Diffusion-hardened medical implant

ABSTRACT

A composition and medical implant made therefrom, the composition including a thick diffusion hardened zone, and preferably further including a ceramic layer. Also provided are orthopedic implants made from the composition, methods of making the composition, and methods of making orthopedic implants from the composition.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of pending U.S. patent applicationSer. No. 15/252,912, filed on Aug. 31, 2016, which is a division of U.S.patent application Ser. No. 14/028,136, filed on Sep. 16, 2013 andissued as U.S. Pat. No. 9,457,126, which is a division of U.S. patentapplication Ser. No. 13/170,084, filed on Jun. 27, 2011 and issued asU.S. Pat. No. 8,647,701, which is a continuation of U.S. patentapplication Ser. No. 12/244,492, filed on Oct. 2, 2008 and issued asU.S. Pat. No. 7,968,209, which s a divisional of U.S. patent applicationSer. No. 11/558,756, filed on Nov. 10, 2006 and issued as U.S. Pat. No.7,550,209, which claims the benefit of U.S. Provisional Application Ser.No. 60/750,557, filed on Dec. 15, 2005, the disclosures of eachapplication hereby incorporated herein by reference in their entirety.

TECHNICAL FIELD

The present invention relates to a new composition of diffusion-hardenedoxidized zirconium. The new composition has application, for example, inarticulating and non-articulating surfaces of medical implants. Thepresent invention also relates to orthopedic implants comprising the newcomposition, methods of making the new composition, and methods ofmaking medical implants comprising the new composition. While thepresent implant composition is useful in hard-on-soft applications(e.g., a medical implant component of the present invention articulatingagainst polyethylene), the present invention also encompasses the use ofthis new medical implant composition in hard-on-hard applications (e.g.,the present composition articulating against itself or against otherhard materials and ceramics) in a hip, knee, spinal, or other implant.

BACKGROUND OF THE INVENTION

Medical implant materials, in particular orthopedic implant materials,must combine high strength, corrosion resistance and tissuecompatibility. The longevity of the implant is of prime importanceespecially if the recipient of the implant is relatively young becauseit is desirable that the implant function for the complete lifetime of apatient. Because certain metal alloys have the required mechanicalstrength and biocompatibility, they are ideal candidates for thefabrication of prostheses. These alloys include 316L stainless steel,chrome-cobalt-molybdenum alloys (Co—Cr), titanium alloys and morerecently zirconium alloys which have proven to be the most suitablematerials for the fabrication of load-bearing and non-load bearingprostheses.

To this end, oxidized zirconium orthopedic implants have been shown toreduce polyethylene wear significantly. The use of diffusion-hardenedoxide surfaces such as oxidized zirconium in orthopedic applications wasfirst demonstrated by Davidson in U.S. Pat. No. 5,037,438. Previousattempts have been made to produce oxidized zirconium coatings onzirconium parts for the purpose of increasing their abrasion resistance.One such process is disclosed in U.S. Pat. No. 3,615,885 to Watson whichdiscloses a procedure for developing thick (up to 0.23 mm) oxide layerson Zircaloy 2 and Zircaloy 4. However, this procedure results insignificant dimensional changes especially for parts having a thicknessbelow about 5 mm, and the oxide film produced does not exhibitespecially high abrasion resistance.

U.S. Pat. No. 2,987,352 to Watson discloses a method of producing ablue-black oxide coating on zirconium alloy parts for the purpose ofincreasing their abrasion resistance. Both U.S. Pat. Nos. 2,987,352 and3,615,885 produce a zirconium oxide coating on zirconium alloy by meansof air oxidation. U.S. Pat. No. 3,615,885 continues the air oxidationlong enough to produce a beige coating of greater thickness than theblue-black coating of U.S. Pat. No. 2,987,352. This beige coating doesnot have the wear resistance of the blue-black coating and is thus notapplicable to many components where there are two work faces in closeproximity. The beige coating wears down more quickly than the blue-blackoxide coating with the resulting formation of oxidized zirconiumparticles and the loss of the integrity of the oxidized zirconiumsurface. With the loss of the oxide surface the zirconium metal is thenexposed to its environment and can lead to transport of zirconium ionsinto the adjacent environment.

The blue-black coatings have a thickness which is less than that of thebeige coating although the hardness of the blue-black coating is higherthan that of the beige coating. This harder blue-black oxide coatinglends itself better to surfaces such as prosthetic devices. Although theblue-black coating is more abrasion resistant than the beige coating itis a relatively thin coating. It is therefore desirable to produce newand improved compositions that maintain the desirable properties of theblue-black coatings of the prior art (for example, increased abrasionresistance).

As discussed above, U.S. Pat. No. 5,037,438 to Davidson discloses amethod of producing zirconium alloy prostheses with a oxidized zirconiumsurface. U.S. Pat. No. 2,987,352 to Watson discloses a method ofproducing zirconium bearings with a oxidized zirconium surface. Theoxide coating produced is not always uniform in thickness and thenon-uniformity reduces the integrity of the bonding between thezirconium alloy and the oxide layer and the integrity of the bondingwithin the oxide layer. Both U.S. Pat. Nos. 2,987,352 and 5,037,438 areincorporated by reference as though fully set forth herein.

In U.S. Pat. Nos. 6,447,550; 6,585,772 and pending U.S. application Ser.No. 10/942,464, Hunter, et al. describe methods for obtaining anoxidized zirconium coating of uniform thickness. Hunter teaches thatsuch is obtained by applying pre-oxidation treatment techniques and bymanipulation of substrate microstructure. The use of uniform thicknessoxide layer results in increased resistance to corrosion by the actionof the body fluids as well as other benefits and is biocompatible andstable over the lifetime of the recipient. U.S. Pat. Nos. 6,447,550;6,585,772 and pending U.S. application Ser. No. 10/942,464 areincorporated by reference as though fully set forth herein.

The oxidized zirconium surfaces of Davidson and Hunter (henceforthreferred as Davidson-type oxidized zirconium composition), while havingrelatively thick ceramic oxide or nitride layers, did not exhibit thickdiffusion hardened zones below the ceramic oxide or nitride. Thediffusion hardened zones of Davidson-type oxidized zirconiumcompositions had thicknesses of at most 1.5-2 microns and typically lessdepending upon the conditions used to produce the composition. FIG. 1shows the nano-hardness profile of Davidson-type oxidized zirconiumcomposition (FIG. 1 is taken from M. Long, L. Reister and G. Hunter,Proc. 24^(th) Annual Meeting of the Society For Biomaterials, Apr.22-26, 1998, San Diego, Calif., USA). The diffusion zone of theDavidson-type oxidized zirconium is between 1.5 to 2 microns. The oxideis approximately 5 microns, hence the totality of the hardened zone inthe Davidson oxide is approximately 7 microns. While the resultingcompositions of Davidson and Hunter exhibited high wear resistance incomparison to those compositions available in the prior art, there isstill room for improvement.

The significant reduction in wear of polyethylene against oxidizedsurfaces is attributed to the ceramic nature of the oxide. The oxidizedzirconium implant typically has a 5 to 6 micron thick ceramic surface(zirconium oxide) that is formed by a thermally driven diffusion processin air. Beneath the zirconium oxide is a hard, oxygen-rich diffusionlayer of approximately 1.5 to 2 microns. The totality of hardened zones(oxide plus diffusion hardened alloy) render the implant resistant tomicroscopic abrasion (for example, from third bodies such as bonecement, bone chips, metal debris, etc.) and slightly less resistant tomacroscopic impact (surgical instrumentation and fromdislocation/subluxation contact with metallic acetabular shells). Thesmaller hardening depth of these implants renders them less than optimalfor hard-on-hard applications. In a hard-on-hard application such as ina hip joint, the material articulates against itself or another hardenedor non-hardened metal instead of polyethylene. The wear rates in suchtypes of implants could be as high as 1 micron per year. With thetotality of the hardened zone (oxide and diffusion zone) having athickness of less than 7 microns, Davidson-type oxidized zirconiumimplants, although representing the state-of-the-art when originallyintroduced and still quite useful, have room for improvement in suchapplications. Hunter et al (U.S. Pat. No. 6,726,725) teaches suchhard-on-hard applications for Davidson-type oxidized zirconiumcomponents. Hunter '725 teaches that the oxide thickness can beincreased up to 20 microns for such applications. But as will be shownherein, Davidson-type oxide compositions having such thicknesses,although highly wear-resistant, can have significant number of oxidelayer defects. Such defects can lead to localized spalling of the oxide.Also, in the Davidson-type composition below the oxide, there is arelatively small diffusion hardened zone. Thus, while the Davidson-typecompositions exhibited superior wear resistance compared to manyconventional materials, there is always room for improvement.

Currently, there are two primary types of hard-on-hard hip implants thatare available commercially, namely metal-on-metal andceramic-on-ceramic. The current standard material of metal-on-metalimplants is high carbon Co—Cr alloy. The major concern with themetal-on-metal implant is the metal ion release from the joint and itsunknown effects on the physiology of the human body. The advantage ofmetal-on-metal implants is that they can be used in larger sizes. Thelarger size of the implant allows greater range of motion. Themetal-on-metal implants have also been shown to be useful forresurfacing type of application where conservation of bone is desired.In such larger joints, the conventional or cross-linked polyethylene isnot preferred and metal-on-metal may be the only choice available. Thelarger size requires polyethylene liner to be thinner. A thinner linermay not be mechanically strong, may creep more or may lead to increasedwear and osteolysis and eventually failure of the implant.

The other commonly used hard-on-hard implant material isceramic-on-ceramic. The current standard material of ceramic-on-ceramicimplants is alumina. Metal ion release is typically not a concern forthese implants. But due to limited toughness and the brittle nature ofceramics, it is difficult to make these implants in larger sizes. Theceramic components have finite probability of fracture thus leading to apotential joint failure and complications associated with the fractureof a joint.

It has been an object of much of the prior art to reduce the metal ionrelease and minimize the fracture risk by combining metal and ceramiccomponents. Fisher et al (U.S. Patent Application 2005/0033442) andKhandkar et al. (U.S. Pat. No. 6,881,229) teach using a metal-on-ceramicarticulation. Fisher et al teach that the difference in hardness betweenthe metallic component and the ceramic component to be at least 4000MPa. Khandkar et. al. specifically teach use of silicon nitride ceramiccomponents for articulating against the metallic component. In bothinstances the objective is to lower the wear of mating couples. But inboth instances, the fracture risk of ceramic is still significant. Theobject of the present invention is to eliminate the risk of fracturealong with metal ion release. It is eliminated by using a metalliccomponent with ceramic surface and diffusion hardened zone below theceramic surface. As mentioned in the details of the invention, diffusionhardened composition of present invention provides a solution to theabove described problems pertaining to hard-on-hard bearings made fromDavidson-type oxidized zirconium, high carbon CoCr (cobalt-chromium) andalumina. In one aspect of invention, the invented composition isapplicable in knee joints and in spinal joints where hard-on-hardarticulation is desired.

Unlike the Davidson-type oxidized zirconium, the oxidized zirconiumcomposition disclosed herein is significantly less susceptible to damagecaused by dislocation and subluxation. Thus, while the application ofdiffusion-hardened oxide layers such as Davidson-type oxidized zirconiumto orthopedic implants represented a great improvement in the art ofimplant materials, resulting in substantial improvements in abrasionresistance and service life, the new compositions of the presentinvention represent improvements over the Davidson-type compositions.

Production of a diffusion hardened zone in zirconium (and its alloys)and titanium (and its alloys) has been disclosed previously. One of theapproach suggested by Kemp (U.S. Pat. No. 5,399,207) is to oxidize azirconium alloy in a temperature range of 426° C. (800° F.) to 871° C.(1600° F.) for two hours or more. The approach of Kemp is to run theprocess longer so that oxygen diffuses farther into the substrate whilethe oxidation is taking place. The major disadvantage of this approachis higher temperature and prolonged time is required to form a thickerdiffusion zone. The higher temperature and prolonged time can lead tomicrostructural changes in the substrate and to a defective oxide thatcomprises substantial amounts of cracks and pores. Kemp teaches theapplication of its method on a Zircadyne 702 substrate. Following theteachings of Kemp, Zircadyne 702 and medical grade Zr-2.5Nb (ASTM F2384)were oxidized at 800° C. The oxide thickness of Zircadyne-702 sampleswas 10 to 12 micron whereas that of Zr-2.5Nb was approximately 20microns (FIGS. 2A and 2B). The diffusion hardened zone on both sampleswas approximately 25 microns (FIG. 2C). The oxide of both samples showedsubstantial defects in the form of cracks and pores.

In another approach. Davidson (U.S. Pat. No. 5,372,660) teachesoxidizing Ti alloy that contains Zr. The presence of Zr in Ti leads toformation of an oxide and a thicker diffusion zone. Following theteachings of Davidson an alloy of Ti—Zr—Nb (55% Ti w/w, 35% Zr w/w and10% Nb w/w) and medical grade Zr-2.5 Nb were oxidized in air. The alloysamples were oxidized at 635° C. for 6 hours. FIG. 3 showsmetallographic images showing the oxide and diffusion hardened zone. Theoxide of both Ti—Zr—Nb and Zr-2.5Nb is cracked. The oxide of Ti—Zr—Nbappears to separate from the substrate at several locations. FIG. 3Cshows micro-hardness of diffusion hardened zone. The Ti—Zr—Nb alloyshows approximately 10 to 15 micron thick diffusion hardened zone. Thediffusion hardened zone of Zr-2.5Nb is less than 5 microns. Thusfollowing the teachings of Kemp and Davidson, a significant depth ofhardening could be obtained but at the cost of substantial defects inthe resulting oxide. Kemp teaches a prolonged treatment at elevatedtemperatures, whereas Davidson teaches changing the chemistry of thealloy to form a thicker diffusion hardened zone. But in both cases theoxide formed is full of defects. Such type of defects in the oxide cancompromise integrity of the oxide and may lead to localized spalling.One of the compositions disclosed herein comprises a thick diffusionzone along with a substantially defect-free oxide. The oxide disclosedherein has additional distinctions over the prior art that will bedisclosed further in the details herein. The Davidson-type and Kemp-typeoxidized zirconium product is an oxide that is predominantly singlephase. The oxide of the present invention comprises a secondary phasethat is ceramic or oxygen-rich metal. Embodiments of the diffusionhardened zone of the present invention have a layered structure and apreferred hardness profile.

Another approach to produce a diffusion hardened metallic zone isbasically one of forming an oxide on the surface of the article bytreatment in an oxygen-rich environment, followed by heat treating thearticle in an oxygen-deficient environment. One of the approachesprovided by Takamura (Trans JIM, vol. 3, 1962, p. 10) has been tooxidize a titanium sample followed by treating it in argon gas (i.e., anoxygen deficient environment with a low partial pressure of oxygen).This apparently allows oxygen to diffuse in the substrate and form athick diffusion zone. Presence of oxygen in the diffusion zone leads tohardening. Another approach suggested by Dong et al (U.S. Pat. No.6,833,197) is to use vacuum or an inert gas mixture to achieve anoxygen-deficient environment, thereby achieving the diffusion-hardeningafter oxidation. The preferred temperature specified by both Takamuraand Dong et al for oxidation is 850° C. and that for diffusion hardening(vacuum treatment) is 850° C. Dong et al suggest this methodology fortitanium and zirconium and titanium/zirconium alloys. One of theproblems with these methods, particularly for zirconium alloys, is thatthe oxidation and diffusion hardening temperatures are significantlyhigh and can lead to thick and cracked (defective) oxide as well ascracks in the substrates after diffusion hardening. Dong demonstratesits method using titanium alloys; no examples forzirconium/niobium-based or titanium/zirconium/niobium-based alloys havebeen shown.

Both Takamura and Dong et. al. recommend a preferred temperature ofoxidation and inert gas/vacuum treatment of 850° C. Following theirteachings, samples of Ti-6Al-4V and medical grade Zr-2.5Nb were oxidizedat 850° C. for 0.3 hr in air. FIGS. 4A and 4B show metallographic imagesafter oxidation. The oxide on the Ti-6Al-4V is less than 1 micron thick.The oxide does not seem to adhere well to the substrate. The oxide onZr-2.5Nb is approximately 12 microns thick and it is cracked. Followingthe teachings of Dong, both samples were subjected to vacuum treatmentunder pressure of 10⁻⁴ torr and at 850° C. for 22 hours. FIGS. 4C and 4Dshow metallographic images after vacuum treatment. In both samples,oxide has dissolved into the substrate. There are no visible cracks inTi-6Al-4V sample. The crack is still present on the surface of theZr-2.5Nb sample. The crack appears to have propagated inside thesubstrate during the vacuum treatment. These types of cracks on thesurface can significantly reduce fatigue strength of the alloy. The newcomposition and method of the present invention overcomes thesedeficiencies.

In order to further demonstrate the difference in the behavior betweenTi and Zr alloys, samples of Ti-6Al-4V and Zr-2.5Nb were oxidized at alower temperature (600° C. for 75 minutes). These samples were thentreated under vacuum (<10⁻⁴ torr) at 685° C. for 10 hours. As will bedisclosed further herein, the treatment was carried out in such a waythat oxide is partially retained on the Zr-2.5Nb substrate. FIGS. 5A and5B show metallographic images of the oxide formed on Ti-6Al-4V andZr-2.5Nb samples. The oxide on Ti-6Al-4V is less than 0.1 micron whereasit is approximately 3 micron on Zr sample. No cracks are visible on bothsamples. After vacuum diffusion hardening, oxide on a Ti-6Al-4V sampleis completely dissolved whereas approximately 1 micron oxide is retainedon a Zr-2.5Nb sample (FIGS. 5C and 5D). Oxygen diffused almost entirelythrough the Ti alloy sample and thus produced a negligibly small depthof hardening whereas it did produce a significant depth of hardening inZr alloy. This example further illustrates the differences in Zr and Tialloys in the Dong process. It is evident from these examples that therange of temperatures that may work for Zr alloys may not be optimal forTi alloys and vice versa. Dong also teaches a sigmoid shaped hardnessprofile of the diffusion hardened metallic zone. The sigmoid shapeddiffusion hardened zone profile requires almost complete dissolution ofthe oxide in the substrate. The inventors of the present invention havefound that this is not necessary. The inventors have found that in oneaspect of this invention, it is advantageous to retain the oxide on thesurface during this process. This is accomplished by careful selectionof temperature and time for oxidation and subsequent diffusionhardening. Dong does not teach or suggest retention of the oxide on thesurface of the sample at the end of the vacuum treatment and obtainingdifferent types of oxygen concentration or hardness profiles other thana sigmoid profile when the oxide is almost completely dissolved.

In another approach of the prior art, Treco (R. Treco, J. Electrochem.Soc., Vol. 109, p. 208, 1962) used vacuum annealing method to completelydissolve the oxide formed on Zircalloy-2 after corrosion testing. Theobjective of Treco's work was to eliminate the oxide by vacuum annealingand the resultant diffusion zone by acid pickling. Treco neitherdiscloses advantage of retaining the oxide during diffusion process nordiscloses an application where such surfaces could be used. Finally,both Dong and Treco do not disclose use of such a technique to form aceramic oxide and diffusion hardened zone to make a damage resistantmedical implant.

The inventors have found that the damage (i.e., wear) resistance ofdiffusion hardened medical implant compositions can be improved byincreasing the thickness of totality of the hardened zones. Theresulting diffusion hardened medical implant compositions are new andnot disclosed or suggested in the prior art. The desired totality ofhardened zones can be achieved by varying the thicknesses of the ceramicoxide (or nitride, or mixed oxide/nitride) and the underlying diffusionhardened zone(s). Additionally, an increase in the thickness of thediffusion hardened zone imparts additional wear resistance desired inhard-on-hard articulation. A thicker diffusion hardened zone exhibits alayered structure in which the concentration of the diffusion hardeningspecies varies with depth. Careful consideration needs to be applied inselecting the temperature and time of oxidation and diffusion hardeningto achieve the desired totality of the hardened zones, while retaining(or enhancing) most of the mechanical, and electrochemical properties ofthe articles. Furthermore, the proper conditions for the processes ofmanufacture of such compositions are related to the alloy system underconsideration. Such hardened alloys are suitable for articulationagainst soft polymers (such as ultra high molecular weight polyethylene(UHMWPE), cross-linked polyethylene (XLPE), polyurethane, etc. and inhard-on-hard bearing applications against like hardened alloys, againstCoCr alloys, ceramics (alumina, silicon nitride, silicon carbide,zirconia, etc.), other hard materials such as diamond, diamond-likecarbon and ceramic coatings (metal-oxides, metal-nitrides,metal-carbides and diamond), etc.

All of the above-referenced U.S. patents and published U.S. patentapplications are incorporated by reference as though fully describedherein.

BRIEF SUMMARY OF THE INVENTION

In one aspect of the present invention there is a medical implantcomprising: a substrate comprising zirconium or zirconium alloy; adiffusion hardened zone in contact with said substrate, said diffusionhardened zone comprising zirconium or zirconium alloy and a diffusionhardening species, said diffusion hardened zone having a thickness ofgreater than 2 microns; and, a substantially defect-free ceramic layerin contact with said diffusion hardened zone and comprising a surface ofsaid medical implant, said ceramic layer ranging in thickness from 0.1to 25 microns; and, wherein the total thickness of the ceramic layer andthe diffusion hardened zone is 5 microns or greater. In someembodiments, the ceramic layer comprises a secondary phase, and thediffusion hardened zone has a layered structure comprising at least twodistinct layers under metallographic analysis, the layered structurecharacterized by: a first layer directly below the ceramic layer,wherein the first layer is predominantly alpha phase zirconium; aninterface between the first layer and the ceramic layer, and; a secondlayer directly below the first layer. In some embodiments, the substratefurther comprises titanium, tantalum, hafnium, niobium, and anycombination thereof. In some embodiments, the diffusion hardeningspecies is selected from the group consisting of oxygen, nitrogen,boron, carbon, and any combination thereof. Preferably, the diffusionhardening species comprises oxygen. In some embodiments, the diffusionhardened zone has a concentration of oxygen which decreases in thedirection of the substrate, said decrease of oxygen concentration beingdefined by a function selected from the group consisting of an errorfunction, an exponential function, a near uniform distribution function,and any sequential combination thereof. In some embodiments, the ceramicoxide has monoclinic content of greater than 93%. In some embodiments,the diffusion hardened zone has a hardness profile which is defined by afunction selected from the group consisting of an error function, anexponential function, a near uniform distribution function, and anysequential combination thereof. In some embodiments, the first layer hasa thickness which is greater than or equal to the thickness of saidsecond layer and of any subsequent layers if present. In someembodiments, the diffusion hardened zone has a thickness of 5 to 70microns. The diffusion hardened zone may have a thickness of 10 to 50microns. The diffusion hardened zone may have a thickness of 15 to 30microns. In some embodiments, the hardness of the diffusion hardenedzone is at least 10% greater than that of the substrate In someembodiments, the medical implant is selected from the group consistingof a hip implant, a knee implant, and a spinal implant. In someembodiments, the substrate comprises an alloy of zirconium and niobiumand has a niobium content of at least 1% (w/w). The substrate maycomprise an alloy of zirconium and niobium has a niobium content of atleast 10% (w/w). In some embodiments, the medical implant furthercomprises an oxygen-containing zirconium alloy overlaying said ceramicoxide or nitride on the surface of said implant, said alloy being in themetallic state.

In another aspect of the present invention there is a medical implantcomprising: a substrate comprising zirconium or zirconium alloy; adiffusion hardened zone in contact with said substrate, said diffusionhardened zone comprising zirconium or zirconium alloy and a diffusionhardening species, said diffusion hardened zone having a thickness ofgreater than 5 microns, and, wherein the diffusion hardened zone has alayered structure comprising at least two distinct layers undermetallographic analysis, said layered structure characterized by: afirst layer on a surface of the implant; a second layer directly belowsaid first layer, wherein said first layer is predominantly alpha phasezirconium; and, said layered structure having a concentration ofdiffusion hardening species which decreases in the direction of thesubstrate, said decrease of concentration of diffusion hardening speciesbeing defined by a function selected from the group consisting of anerror function, an exponential function, a near uniform distributionfunction, and any sequential combination thereof. In some embodiments,the substrate further comprises titanium, tantalum, hafnium, niobium,and any combination thereof. In some embodiments, the diffusionhardening species is selected from the group consisting of oxygen,nitrogen, boron, carbon, and any combination thereof. Preferably, thediffusion hardening species comprises oxygen. In some embodiments, thediffusion hardened zone has a concentration of oxygen which decreases inthe direction of the substrate, said decrease of oxygen concentrationbeing defined by a function selected from the group consisting of anerror function, an exponential function, a near uniform distributionfunction, and any sequential combination thereof. In some embodiments,the diffusion hardened zone has a hardness profile which is defined by afunction selected from the group consisting of an error function, anexponential function, a near uniform distribution function anysequential combination thereof. In some embodiments, the first layer hasa thickness which is greater than the thickness of said second layer andof any subsequent layers if present. In some embodiments, the diffusionhardened zone has a thickness of 5 to 70 microns. The diffusion hardenedzone may have a thickness of 10 to 50 microns. The diffusion hardenedzone may have a thickness of 15 to 30 microns. In some embodiments, thehardness of the diffusion hardened zone is at least 10% greater thanthat of the substrate. In some embodiments, the medical implant isselected from the group consisting of a hip implant, a knee implant, anda spinal implant. In some embodiments, the substrate comprises an alloyof zirconium and niobium has a niobium content of at least 1% (w/w). Thesubstrate may comprise an alloy of zirconium, titanium and niobium andhas a niobium content of at least 10% (w/w).

In another aspect of the present invention there is a method of making asurface hardened medical implant comprising the steps of: forming saidmedical implant of zirconium or zirconium alloy; and, further treatingsaid implant by any one of (a), (b), or (c), wherein (a), (b), and (c)are defined as follows: (a) treating said implant in the presence ofceramic-forming species at a temperature of less than 700° C. forgreater than 5 minutes; and, thereafter treating said implant undervacuum or inert gas at a temperature of from 500° C. to 1000° C. forgreater than 1 hour; (b) treating said implant in the presence ofceramic-forming species at a temperature of from 500° C. to 1000° C.;and, thereafter treating said implant under vacuum or inert gas at atemperature less than 700° C.; (c) treating said implant in the presenceof ceramic-forming species at a temperature of less than 700° C.; and,thereafter treating said implant under vacuum or inert gas at atemperature less than 700° C. In some embodiments, the method furthercomprises the step of treating said implant in the presence of aceramic-forming species at a temperature less than 700° C. for greaterthan 5 minutes after said step of thereafter treating said implant undervacuum or inert gas. In some embodiments, the step of thereaftertreating said implant under vacuum or inert gas is performed at atemperature of 600° C. to 700° C. In some embodiments, the step oftreating said implant in the presence of ceramic-forming species isperformed for between 5 minutes to 12 hours. In some embodiments, thestep of thereafter treating said implant under vacuum or inert gas isperformed for between 15 minutes to 30 hours. In some embodiments, thestep of forming a medical implant of zirconium or zirconium alloycomprises forming said medical implant of zirconium alloy having analloying element selected from the group consisting of titanium,tantalum, hafnium, niobium, and any combination thereof. In someembodiments, the step of forming comprises forming said medical implantof an alloy of zirconium and niobium, said alloy having a niobiumcontent of at least 1% (w/w). In some embodiments, the step of formingcomprises forming said medical implant of an alloy of zirconium andniobium, said alloy having a niobium content of at least 10% (w/w). Insome embodiments, the step of treating said implant in the presence ofceramic-forming species and said step of thereafter treating saidimplant under vacuum or inert gas comprise treating said implant with adiffusion hardening species selected from the group consisting ofoxygen, nitrogen, boron, carbon, and any combination thereof.

In another aspect of the present invention there is a method of makingsurface hardened medical implant comprising steps of: forming saidmedical implant of zirconium or zirconium alloy; forming an oxide,carbide, nitride, boride or combination thereof, on a surface of saidimplant at a temperature of from 500° C. to 1000° C. for greater than 2hours: removing the formed oxide, carbide, nitride, boride, orcombination thereof; and, thereafter re-forming an oxide, carbide,nitride, boride, or combination thereof, on a surface of said implant ata temperature of from 500° C. to 1000° C. for greater than 5 minutes.

In another aspect of the present invention there is a method of makingsurface hardened medical implant comprising steps of: forming saidmedical implant of zirconium or zirconium alloy; diffusing oxygen ornitrogen into said implant at a partial pressure of oxygen or nitrogenof less than 0.05 bar and at a temperature ranging from 500° C. to 1000°C. for greater than 2 hours; and, thereafter oxidizing or nitriding theimplant between 500° C. to 1000° C. for greater than 10 minutes.

In another aspect of the present invention there is a method of making asurface hardened medical implant comprising the steps of: forming saidmedical implant of zirconium or zirconium alloy; oxidizing or nitridingsaid implant at a temperature of from 500° C. to 700° C. to form atleast a 2 micron thick oxide or nitride; and, thereafter treating saidimplant under vacuum or inert gas at a temperature less than 700° C. toretain at least 0.1 microns oxide, to form at least 0.005 micronsmetallic hardened layer, and to form a diffusion zone having a thicknessof at least 2 microns. In some embodiments, the substrate furthercomprises titanium, tantalum, niobium, hafnium, and any combinationthereof. In some embodiments, the oxide or nitride thickness before saidstep of thereafter treating said implant under vacuum or inert gas isfrom 2 to 15 microns. In some embodiments, the oxide or nitridethickness after said step of thereafter treating said implant undervacuum or inert gas is from 0.1 to 10 microns. In some embodiments, thediffusion hardened zone is from 2 to 50 microns.

In another aspect of the present invention there is a medical implantproduced by the process comprising the steps of: forming said medicalimplant of zirconium or zirconium alloy; further treating said implantby any one of (a), (b), or (c), wherein (a), (b), and (c) are defined asfollows: (a) treating said implant in the presence of ceramic-formingspecies at a temperature of less than 700° C. for greater than 5minutes; and, thereafter treating said implant under vacuum or inert gasat a temperature of from 500° C. to 1000° C. for greater than 1 hour;(b) treating said implant in the presence of ceramic-forming species ata temperature of from 500° C. to 1000° C.; and, thereafter treating saidimplant under vacuum or inert gas at a temperature less than 700° C.;(c) treating said implant in the presence of ceramic-forming species ata temperature of less than 700° C.; and, thereafter treating saidimplant under vacuum or inert gas at a temperature less than 700° C.

In another aspect of the present invention there is a medical implant,comprising: (a) a first implant portion comprising zirconium orzirconium alloy, said first implant portion having a bearing surface;(b) a second implant portion comprising zirconium or zirconium alloy,said second implant portion having bearing surface; (c) wherein thebearing surface of said first implant portion and the bearing surface ofsaid second implant portion each have a size and shape to engage orcooperate with one another; (d) a diffusion hardened zone in contactwith at least a portion of said zirconium or zirconium alloy, saiddiffusion hardened zone forming at least a part of the bearing surfaceof both of said first and second implant portions, said diffusionhardened zone comprising zirconium or zirconium alloy and a diffusionhardening species, said diffusion hardened zone having a thickness ofgreater than 2 microns; and, (e) a substantially defect-free ceramiclayer in contact with said diffusion hardened zone and comprising asurface of said medical implant, said ceramic layer ranging in thicknessfrom 0.1 to 25 microns; wherein the total thickness of the ceramic layerand the diffusion hardened zone is 5 microns or greater. In someembodiments, the ceramic layer comprises a secondary phase; and, thediffusion hardened zone has a layered structure comprising at least twodistinct layers under metallographic analysis, the layered structurecharacterized by: a first layer directly below the ceramic layer,wherein the first layer is predominantly alpha phase zirconium; aninterface between the first layer and the ceramic layer; and: a secondlayer directly below the first layer. In some embodiments, the substratefurther comprises titanium, tantalum, hafnium, niobium, and anycombination thereof. In some embodiments, the diffusion hardeningspecies is selected from the group consisting of oxygen, nitrogen,boron, carbon, and any combination thereof. Preferably, the diffusionhardening species comprises oxygen. In some embodiments, the diffusionhardened zone has a concentration of oxygen which decreases in thedirection of the substrate, said decrease of oxygen concentration beingdefined by a function selected from the group consisting of an errorfunction, an exponential function, a near uniform distribution function,and any sequential combination thereof. In some embodiments, the ceramicoxide has monoclinic content of greater than 93%. In some embodiments,the diffusion hardened zone has a hardness profile which is defined by afunction selected from the group consisting of an error function, anexponential function, a near uniform distribution function and anysequential combination thereof. In some embodiments, the first layer hasa thickness which is greater than or equal to the thickness of saidsecond layer and of any subsequent layers if present. In someembodiments, the diffusion hardened zone has a thickness of 5 to 70microns. Tithe diffusion hardened zone may have a thickness of 10 to 50microns. The diffusion hardened zone may have a thickness of 15 to 30microns. In some embodiments, the hardness of the diffusion hardenedzone is at least 10% greater than that of the substrate. In someembodiments, the medical implant is selected from the group consistingof a hip implant, a knee implant, and a spinal implant. In someembodiments, the substrate comprises an alloy of zirconium and niobiumand has a niobium content of at least 1% (w/w). In some embodiments, thesubstrate comprises an alloy of zirconium and niobium has a niobiumcontent of at least 10% (w/w). In some embodiments, the medical implantfurther comprises an oxygen-containing zirconium alloy overlaying saidceramic oxide or nitride on the surface of said implant, said alloybeing in the metallic state.

In another aspect of the present invention, there is medical implant,comprising: (a) a first implant portion comprising zirconium orzirconium alloy, said first implant portion having a bearing surface;(b) a second implant portion comprising zirconium or zirconium alloy,said second implant portion having bearing surface; (c) wherein thebearing surface of said first implant portion and the bearing surface ofsaid second implant portion each have a size and shape to engage orcooperate with one another; (d) a diffusion hardened zone in contactwith at least a portion of said zirconium or zirconium alloy, saiddiffusion hardened zone forming at least a part of the bearing surfaceof both of said first and second implant portions, said diffusionhardened zone comprising zirconium or zirconium alloy and a diffusionhardening species, said diffusion hardened zone having a thickness ofgreater than 5 microns; wherein the diffusion hardened zone has alayered structure comprising at least two distinct layers undermetallographic analysis, said layered structure characterized by: afirst layer on a surface of the implant; a second layer directly belowsaid first layer, wherein said first layer is predominantly alpha phasezirconium; and, said diffusion hardened zone having a concentration ofdiffusion hardening species which decreases in the direction of thesubstrate, said decrease of concentration of diffusion hardening speciesbeing defined by a function selected from the group consisting of anerror function, an exponential function, a near uniform distributionfunction, and any sequential combination thereof. In some embodiments,the substrate further comprises titanium, tantalum, hafnium, niobium,and any combination thereof. In some embodiments, the diffusionhardening species is selected from the group consisting of oxygen,nitrogen, boron, carbon, and any combination thereof. Preferably, thediffusion hardening species comprises oxygen. In some embodiments, thediffusion hardened zone has a concentration of oxygen which decreases inthe direction of the substrate, said decrease of oxygen concentrationbeing defined by a function selected from the group consisting of anerror function, an exponential function, a near uniform distributionfunction, and any sequential combination thereof. In some embodiments,the diffusion hardened zone has a hardness profile which is defined by afunction selected from the group consisting of an error function, anexponential function, a near uniform distribution function and anysequential combination thereof. In some embodiments, the first layer hasa thickness which is greater than the thickness of said second layer andof any subsequent layers if present. In some embodiments, the diffusionhardened zone has a thickness of 5 to 70 microns. The diffusion hardenedzone may have a thickness of 10 to 50 microns. The diffusion hardenedzone may have a thickness of 15 to 30 microns In some embodiments, thehardness of the diffusion hardened zone is at least 10% greater thanthat of the substrate. In some embodiments, the medical implant isselected from the group consisting of a hip implant, a knee implant, anda spinal implant. In some embodiments, the substrate comprises an alloyof zirconium and niobium has a niobium content of at least 1% (w/w). Insome embodiments, the substrate comprises an alloy of zirconium,titanium and niobium and has a niobium content of at least 10% (w/w).

In another aspect of the present invention, there is a medical implantcomprising: (a) a first implant portion, said first implant portionhaving a bearing surface; (b) a second implant portion, said secondimplant portion having a bearing surface; (c) wherein the bearingsurface of said first implant portion and the bearing surface of saidsecond implant portion each have a size and shape to engage or cooperatewith one another, (d) wherein one or both of the two portions of themedical implant comprises a biocompatible alloy having an elasticmodulus less than 200 GPa; and, (e) wherein the difference in radius ofthe mating portions is greater than about 50 microns. In someembodiments, one or both of said first implant portion and said secondimplant portion further comprises: a substrate; a diffusion hardenedzone in contact with said substrate, said diffusion hardened zonecomprising a diffusion hardening species, said diffusion hardened zonehaving a thickness of greater than 2 microns; and, a substantiallydefect-free ceramic layer in contact with said diffusion hardened zoneand comprising a surface of said medical implant, said ceramic layerranging in thickness from 0.1 to 25 microns; and, wherein the totalthickness of the ceramic layer and the diffusion hardened zone is 5microns or greater. In some embodiments, one or both of said firstimplant portion and said second implant portion further comprises: theceramic layer comprises a secondary phase; and, the diffusion hardenedzone has a layered structure comprising at least two distinct layersunder metallographic analysis, the layered structure characterized by: afirst layer directly below the ceramic layer; an interface between thefirst layer and the ceramic layer; and; a second layer directly belowthe first layer. In some embodiments, one or both of said first implantportion and said second implant portion further comprises: a substrate;a diffusion hardened zone in contact with said substrate, said diffusionhardened zone comprising a diffusion hardening species, said diffusionhardened zone having a thickness of greater than 5 microns; and, whereinthe diffusion hardened zone has a layered structure comprising at leasttwo distinct layers under metallographic analysis, said layeredstructure characterized by: a first layer on a surface of the implant; asecond layer directly below said first layer; and, said diffusionhardened zone having a concentration of diffusion hardening specieswhich decreases in the direction of the substrate, said decrease ofconcentration of diffusion hardening species being defined by a functionselected from the group consisting of an error function, an exponentialfunction, a near uniform distribution function, and any sequentialcombination thereof. In some embodiments, one or both of said firstimplant portion and said second implant portion further comprises: asubstrate: a diffusion hardened zone in contact with said substrate,said diffusion hardened zone comprising a diffusion hardening species,said diffusion hardened zone having a thickness of greater than 2microns; and, a substantially defect-free ceramic layer in contact withsaid diffusion hardened zone and comprising a surface of said medicalimplant, said ceramic layer ranging in thickness from 0.1 to 25 microns;and, wherein the total thickness of the ceramic layer and the diffusionhardened zone is 5 microns or greater. In some embodiments, one or bothof said first implant portion and said second implant portion furthercomprises: the ceramic layer comprises a secondary phase; and, thediffusion hardened zone has a layered structure comprising at least twodistinct layers under metallographic analysis, the layered structurecharacterized by: a first layer directly below the ceramic layer; aninterface between the first layer and the ceramic layer; and; a secondlayer directly below the first layer.

The foregoing has outlined rather broadly the features and technicaladvantages of the present invention in order that the detaileddescription of the invention that follows may be better understood.Additional features and advantages of the invention will be describedhereinafter which form the subject of the claims of the invention. Itshould be appreciated by those skilled in the art that the conceptionand specific embodiment disclosed may be readily utilized as a basis formodifying or designing other structures for carrying out the samepurposes of the present invention. It should also be realized by thoseskilled in the art that such equivalent constructions do not depart fromthe spirit and scope of the invention as set forth in the appendedclaims. The novel features which are believed to be characteristic ofthe invention, both as to its organization and method of operation,together with further objects and advantages will be better understoodfrom the following description when considered in connection with theaccompanying figures. It is to be expressly understood, however, thateach of the figures is provided for the purpose of illustration anddescription only and is not intended as a definition of the limits ofthe present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, reference isnow made to the following descriptions taken in conjunction with theaccompanying drawing, in which:

FIG. 1 shows the hardness profile of Davidson-type oxidized zirconiumcomposition. The thickness of the diffusion zone is 1.5 to 2 microns(Long et. al.).

FIGS. 2A and 2B are metallographic images of Zircadyne 702 and Zr-2.5Nboxidized following the teachings of Kemp; FIG. 2C micro-hardness profileof the diffusion hardened zone.

FIGS. 3A and 3B are metallographic images of Ti—Zr—Nb and Zr-2.5Nboxidized by following teachings of Davidson: FIG. 3C Micro-hardnessprofile of diffusion hardened zone.

FIGS. 4A and 4B show samples of Ti-6Al-4V and Zr-2.5Nb oxidized at 850°C. for 0.3 hours respectively; FIGS. 4C and 4D show samples of Ti-6Al-4Vand Zr-2.5Nb diffusion hardened at 850° C. for 22 hours respectively.

FIGS. 5A and 5B show samples of Ti-6Al-4V and Zr-2.5Nb oxidized at 600°C. for 75 minutes respectively; FIGS. 5C and 5D show samples ofTi-6Al-4V and Zr-2.5Nb diffusion hardened at 685° C. for 10 hoursrespectively.

FIG. 6 shows hardness profiles obtained on Zr-2.5Nb samples after vacuumdiffusion process (685° C. for 10 hours). The starting oxide representsoxide thickness prior to vacuum diffusion treatment. The oxidation wascarried out at 635° C. for different times to produce different startingoxide thickness.

FIG. 7 shows metallographic images of samples with hardness profileobtained in FIGS. 3A-3C were re-oxidized at 635° C. for 60 minutes.

FIGS. 8A and 8B illustrate Rockwell indents showing the damageresistance of Davidson-type oxidized zirconium composition and FIGS. 8Cand 8D show damage produced on one of the composition disclosed in thisinvention with a total hardening depth of 20 to 25 microns.

FIG. 9 shows wear results of pin-on-disk testing of high carbon castCoCr against itself and one of the oxidized zirconium compositionsagainst itself (total hardened zone 20 to 25 microns) disclosed in thisinvention.

FIG. 10 shows the oxygen concentration profile of the diffusion zone.Analyses were carried out using a scanning auger microprobe withaccelerating voltage of 10 kV; probe current of 18 nA and electron beamat 30° from sample normal. Oxide was retained on the sample after thevacuum treatment.

FIG. 11 illustrates the micro-hardness profile of Davidson-type oxidizedzirconium composition and some of the compositions disclosed in thisinvention. Micro-hardness was carried out using a Knoop indenter at aload of 10 g.

FIGS. 12A-12D shows cross-sectional metallographic images; FIG. 12ADavidson-type oxidized zirconium composition, FIG. 12B oxidized at 635°C. for 75 minutes and diffusion hardened at 585° C. for 10 hours, FIG.12C oxidized at 690° C. for 60 minutes and diffusion hardened at 685° C.for 20 hours, and FIG. 12D oxidized at 635° C. for 75 minutes anddiffusion hardened at 750° C. for 20 hours. The dotted lines on theimages show the demarcation of layers.

FIG. 13A shows XRD pattern of Davidson-type oxidized zirconium and FIG.13B one of the compositions of this invention. The M (−111) and M (111)are from −111 and 111 plane, T (111) is from tetragonal 111 plane. The T(111) peak for new composition is negligible indicating smallertetragonal phase in the oxide compared to the oxide of Davidson-typeoxidized zirconium. The monoclinic phase analysis was carried using ASTMF 1873.

FIGS. 14A and 14B show a Davidson-type oxidized zirconium composition;FIGS. 14C and 14D show one of the compositions of this invention. Thesample shown in FIGS. 14C and 14D was oxidized at 690° C. for 60 minutesand diffusion hardened at 685° C. for 20 hours. The oxide was retainedon the surface. This is a longitudinal cross-section of the sample. Theorientation of secondary phase is different in transverse section. Adotted line is drawn to show how far the secondary phase is present inthe oxide. The samples are imaged using back scattered electron modewith accelerating voltage of 20 kV.

FIG. 15A illustrates an oxide of Davidson-type oxidized zirconiumcomposition, and FIG. 15B an oxide of the present invention. The brightwhite areas in image FIG. 15B are secondary phase.

FIG. 16 shows the ratio of atomic concentration of oxygen to atomicconcentration of zirconium of Davidson-type oxidized zirconiumcomposition and that disclosed in this invention. The depth profileanalysis was carried out using x-ray photoelectron spectroscope (Al kα,take off angle 45°) and an ion gun for sputtering (Ar+, 3 keV, silicasputter rate of 48 angstroms/minute).

FIGS. 17A and 17B illustrate an error function fit to the micro-hardnessindents in the diffusion hardened zone to estimate the depth ofhardening. The diffusivity values are in cm²/s and are approximate. Timeis in seconds and distance is in microns.

FIG. 18A illustrates the microstructure as received Zr-2.5Nb bar stock,FIG. 18B oxidized at 635° C. for 75 minutes and diffusion hardened at585° C. for 10 hours, FIG. 18C oxidized at 690° C. for 60 minutes anddiffusion hardened at 685° C. for 20 hours, and FIG. 18D oxidized at635° C. for 75 minutes and diffusion hardened at 750° C. for 20 hours,and FIG. 18E oxidized at 850° C. for 20 minutes and diffusion hardenedat 850° C. for 22 hours. The samples were polished using standardmetallographic techniques and were heat tinted to reveal the grain size.

DETAILED DESCRIPTION OF THE INVENTION

As used herein, “a” or “an” means one or more. Unless otherwiseindicated, the singular contains the plural and the plural contains thesingular.

As used herein, “zirconium alloy” is defined broadly, and includesalloys having at least 5% (w/w) zirconium. The alloys can be ofzirconium, titanium, hafnium and niobium. The alloys can bepolycrystalline or amorphous or single crystals or combinations of same.

As used herein, “ceramic” is defined as a chemical compound of a metal(or a metal constituent in an alloy) and one or more non-metals,including carbon, oxygen, nitrogen, boron, and combinations thereof.While the preferred embodiment of the ceramic of the present inventionis an oxide, the ceramic of the present invention includes oxides,carbides, nitrides, borides, and any combination thereof. As usedherein. “ceramic layer” is defined as a stratum of material consistingof ceramic which forms a part of a greater material. As used herein, theterm “ceramic coating” refers to a surface transformed layer, surfacefilm, surface oxide, nitride, carbide, boride (or combination thereof)present on the alloy or metal substrate.

As used herein, “ceramic-forming species” is defined as oxygen, carbon,nitrogen, boron, and any combination thereof. It is preferable that theceramic-forming species be in the gas phase during the formation of theceramic layer, although it is possible and within the scope of thepresent invention wherein the ceramic-forming species is present in aphase other than the gas phase. One non-limiting example of a non-gasphase embodiment is wherein the ceramic-forming species is in the solidphase in contact with the substrate to which it is to be introduced. Theceramic-forming species, in addition to forming a ceramic, also acts asa diffusion hardening species in the formation of a diffusion zone.

The “diffusion zone” is defined as the zone below the ceramic surface(if a ceramic surface is present) or at the surface itself (if a ceramicsurface is not present) and that comprises a diffusion hardeningspecies. “Diffusion hardening species” is defined as carbon, oxygen,nitrogen, boron, or any combination thereof. The “diffusion hardenedzone” is defined as that portion of the diffusion zone having hardnessat least 1.1 times greater than the substrate hardness.

As used herein, “biocompatible alloy” is defined as the alloycombinations that are currently used in orthopedic industry. Examples ofsuch alloys include cobalt-chromium-molybdenum,titanium-aluminum-vanadium, nickel-titanium and zirconium-niobium. Theother biocompatible alloys that are referred in this invention are thealloys that are made from either zirconium or titanium or tantalum orniobium or hafnium or combination thereof.

As used herein, the term “vacuum” refers to a pressure of less thanabout 10⁻² torr.

Implants comprising Davidson-type oxidized zirconium have been shown toreduce polyethylene wear significantly. This significant reduction inwear is attributed to its ceramic surface. The oxidized zirconiumimplant typically has 4 to 5 micron thick ceramic surface (zirconiumoxide) that is formed by a thermally driven diffusion process in air.Beneath the zirconium oxide is a hard, oxygen-rich diffusion layer ofapproximately 1.5 to 2 microns. The totality of hardened zones (oxideplus diffusion hardened alloy) render the implant resistant tomicroscopic abrasion (third bodies such as bone cement, bone chips,metal debris, etc.) and slightly less resistant to macroscopic impact(surgical instrumentation and from dislocation/subluxation contact withmetallic acetabular shells). However, like all conventional medicalimplant materials, Davidson-type oxidized zirconium implants aresusceptible to damage caused by dislocation and subluxation(macroscopic). Although not intending to be bound by theory, it isbelieved that this susceptibility is due to the relatively smallthickness of the total hardened zones (5 micron oxide plus 1.5 to 2micron diffusion zone) in the Davidson-type oxidized zirconium products.Although Davidson-type oxidized zirconium implants perform better thanmost materials in hard-on-soft applications, the small hardened zone isnot ideal for hard-on-hard bearing applications. The abrasion resistanceof oxidized zirconium and other common implant alloys can be improved byincreasing the depth of totality of the hardened zones. Such hardenedalloys are suitable for articulation against soft polymers (such asUHMWPE, XLPE, polyurethane, etc.) and in hard-on-hard bearingapplications against like hardened alloys, against CoCr alloys, ceramics(alumina, silicon nitride, silicon carbide, zirconia, etc.), and otherhard materials such as diamond, diamond-like carbon, etc.

FIG. 6 shows four types of hardness profiles obtained on Zr-2.5Nb alloysamples using an embodiment of the method of the present invention. Thefour profiles obtained are Profile 1: uniform function. Profile 2: acombination of uniform function and exponential function, Profile 3: acombination of exponential function and error function, Profile 4: errorfunction. As will be discussed in detail, the resultant shape of thehardness profile was carefully controlled by the oxide thickness,oxidation and vacuum treatment temperatures and time. In this particularexample, the starting oxide thickness was varied by varying oxidationtime at a constant temperature of 635° C. Samples were oxidized for 5minutes, 15 minutes, 30 minutes and 60 minutes respectively. All thesamples were vacuum treated at 685° C. for 10 hours. After vacuumtreatment the four samples produced four different profiles as shown inFIG. 6. The oxide was retained on sample with profile 4 and wascompletely dissolved on samples with profiles 1 to 3. Each of theseprofiles can have a distinct advantage over the other. For example, ifthe oxidation step needs to be repeated after vacuum treatment to formoxide, then Profiles 1 to 3 may produce a high integrity predominantlydefect-free oxide compared to Profile 4. FIG. 7 shows metallographicimages of the oxide formed on samples with different profiles. Thesesamples were oxidized after the vacuum treatment at 635° C. for 1 hourto produce 5 to 6 micron thick oxide. As can be seen, oxide on theProfile 4 is cracked and non-uniform compared to that formed in Profiles1 to 3. This is believed to be caused by lack of plasticity of thediffusion hardened zone that cannot accommodate stresses generatedduring re-oxidation. This example illustrates another embodiment of theinvention that will be disclosed. If re-oxidation of the alloy samplesis desired after diffusion hardening process, it is important to obtainan adequate diffusion profile (Profiles 1 to 3). The appropriatediffusion profile ensures a substantially defect-free oxide formationafter the vacuum treatment. The oxidation process is typicallyaccompanied by the volume expansion of the surface (oxide). If thestresses generated during volume expansion are not accommodated in thesubstrate, it can lead to defects such as cracks and pores in the oxide.An example of such defects in the oxide is shown in FIG. 7 (Profile 4).Cracks and pores can compromise integrity of the oxide and may lead tospalling of the oxide. Another type of defect that is anticipated inthis disclosure is the uniformity of the oxide-metal interface. FIG. 7shows an example of wavy interface formed on samples of Profile 3. Thereare few pores and cracks but there are areas where the oxide thicknessis less than 50% of the nominal oxide thickness. Such type of wavyinterface may be unacceptable for a medical implant since there is apotential compromise of the integrity of the oxide.

In a medical implant application, it is desirable that the oxide (orother ceramic layer) formed is substantially defect-free. When the oxideis formed on zirconium alloy substrate, there is expansion of volume asoxygen atoms are added in the zirconium matrix. This volume expansionleads to significant amount of stresses that need to be dissipated. Ifthe substrate underneath is significantly brittle to start with, poresand cracks may form in the oxide to dissipate the stresses. It may alsolead to a wavy interface between the oxide and metal. It sometimes maylead to spalling of the oxide as well. The defects in the oxide can bebroadly classified as pores and cracks. The pores can be circular orelongated and may be on the surface or at the interface. The cracks canbe perpendicular to the oxide metal interface, and/or may be parallel tothe oxide metal interface. Another type of defect that is anticipated inthis disclosure is the wavy oxide metal interface and delaminated orspalled oxide. One object of the present invention is to produce asubstantially defect-free ceramic layer with a thicker diffusionhardened zone. As mentioned previously, following the prior artteachings of Kemp and Davidson, a thicker diffusion zone can be obtainedbut it produces an oxide that is not substantially defect-free. Forexample, FIG. 2A shows that the oxide is separated from the oxide metalinterface. FIG. 2B shows a crack perpendicular to the oxide metalinterface. FIG. 7 (profile 4) shows oxide with several elongated pores,and cracks that are parallel to the interface. FIG. 7 (profile 3) showsan example of another type of defect where the oxide metal interface iswavy. It is the object of this invention to form a ceramic layer that issubstantially free of such defects. The defects in the ceramic layer areevaluated on a cross-sectional metallographic sample at 1000×magnification with field of view of approximately 100×80 microns. Thesubstantially defect-free ceramic layer of the present invention ischaracterized by a) average pore diameter smaller than 15% of ceramiclayer thickness, b) average crack length parallel to the ceramiclayer/metal interface to be less than 25% of ceramic layer thickness,(c) average opening width of crack perpendicular to the ceramiclayer/metal interface to be less than 15% of ceramic layer thickness and(d) the difference between average and minimum ceramic layer thicknessto be less than 50% of the nominal oxide thickness. It is possible thatthe all defects described above may appear in one field of view or onlyfew of them in one view and all remaining in another view. Thedefect-free ceramic layer of the present invention is defined as that inwhich above mentioned defects are not seen in at least 3 out of 5 fieldsof randomly chosen views. The ceramic layer which is substantially freeof such defects is termed as defect-free.

In the present invention, there is medical implant and a method ofproducing the medical implant; the medical implant having a defect-freeceramic layer comprising a secondary phase along with diffusion hardenedzone underneath the ceramic layer. This is accomplished by carefulcontrol of the ceramic formation and diffusion hardening temperatures.In one aspect of this invention, this leads to a preferred profile ofthe hardened zone beneath the ceramic layer. In another aspect of theinvention, the ceramic layer is preferentially retained on the surfaceand is comprised of a secondary phase. In another aspect of invention,an adequate hardness profile is obtained if re-formation of the ceramiclayer is required after diffusion hardening. In another aspect of thepresent invention, the diffusion zone is comprised of a layeredstructure. In another aspect of the present invention, a hardenedmetallic film is formed on the surface of the ceramic layer.

The effect of the hardened zone on damage tolerance was evaluated by aRockwell indent and by carrying out a wear test. FIG. 8 shows backscattered electron images of the indents on Davidson-type oxidizedzirconium composition and that disclosed in this invention. The damagewas produced on a flat disk by indenting the surface with a Rockwellindenter (diamond) with a load of 150 lbf. FIGS. 8A and 8B show damageproduced on the Davidson-type oxidized zirconium composition. It shouldbe noted that applied stress is much greater than that expected in thebody. The indent has caused the oxide to crack in circumferential and inradial direction. The bright area in the center is exposed Zr-2.5Nbsubstrate. The grayish area is oxide. Due to the amount of straininduced during indentation, oxide at the edges of the indent is crackedand removed along with the substrate material. FIGS. 8C and 8D showdamage produced on one of the compositions of the present invention.This sample was oxidized at 635° C. for 75 minutes and then diffusionhardened at 685 C. for 10 hours at a pressure of 10⁻⁴ torr. The oxide(approximately 4 micron thick) was retained on this sample. The hardenedmetallic layer formed on the surface was removed by diamond polishingbefore the test. The total hardened zone of this sample is 20 to 25microns. The damage on this sample is significantly less for the newcomposition than it is for the Davidson-type oxidized zirconiumcomposition. Less amount of substrate Zr-2.5Nb is exposed at the center.The ceramic layer is not removed along the edges of the sample. Althoughthe Davidson-type oxidized zirconium composition was a great advance formedical implants and continues to be superior to other conventionalmedical materials, this example shows the marked improvement in thedamage resistance obtained over the Davidson-type oxidized zirconiumcompositions. FIG. 9 shows results of a wear study when a composition ofthe present invention (in this case, a ceramic oxide) was articulatedagainst itself in a pin on disk test. The test was run on a pin on disktester at an applied load of 10 N for 1 Mcycle. Load was increased to50N at approximately 0.5 Mcycles. Lactated ringer's solution was used asthe test medium. The disks were flat and the pins had 100 mm radius. Thedisks and pins of Zr-2.5Nb were oxidized at 635° C. for 120 minutes andthen diffusion hardened at 685° C. for 10 hours. The oxide(approximately 7 microns) was retained after diffusion hardeningprocess. The metallic layer and part of the oxide was removed by diamondpolishing before the test. The pins were used in as diffusion hardenedcondition and comprised of metallic hardened layer over the oxide andthe layered diffusion zone underneath the oxide. A comparison was alsomade to the current standard of hard-on-hard bearings, high carbon castCoCr. The wear of the new composition was approximately 34 times lessthan that of CoCr against CoCr couple. The pin-on disk test does nottake into account the geometrical constraints encountered in a hip, kneeor spinal joint. Another object of the present invention is to alsoaccount for the geometrical aspects of the joint. It is well-known thatwear in a hard-on-hard hip joint is biphasic. The first phase of wear isa run-in wear and the second phase is steady-state wear. In the run-inphase, the asperities of the mating components wear out. After therunning in wear, based on the component geometry and the stiffness ofthe components, a fluid film is formed between the mating components.This is typically termed as steady-state wear. The steady-state wear istypically less than run-in wear. One of the approaches to reduce therun-in and steady-state wear is to use metal-ceramic articulation astaught by Fisher et al (U.S. Patent Application 2005/0033442) andKhandkar et al (U.S. Pat. No. 6,881,229). Although this will reducemetal-ion release, the fracture risk of the ceramic component stillprevails.

In another approach, Lippincott and Medley (U.S. Pat. No. 6,059,830)teach applying geometrical constraints to the mating hip components. The'830 patent teaches the use of components such that the radiusdifference of the mating components is less than 50 microns. This smalldifference in radius will promote thicker fluid film formation and thusreduced wear of mating metallic components. The disadvantage of thismethod is that a sophisticated manufacturing set-up is required toproduce components with such tight tolerances. The inventors of thepresent invention have found that such a demanding manufacturingapproach is not necessary. A thicker fluid film can also be formed byusing lower elastic modulus (E) alloys such as, for example, Zr and/orTi alloys (having, for example, E<120 GPa), instead of using higherelastic modulus alloys such as CoCr alloys (having, for example, Etypically greater than 200 GPa). This allows for other metal and metalalloy systems (other than zirconium and/or titanium) to be used in thepresent invention as a substrate of the medical implant when the elasticmodulus of such metal and metal alloy systems is less than 200 GPa. Inone aspect of invention, the radial difference between the matingcomponents of the present invention is kept above 50 microns and basedon the radius of the component used can be as high as 150 microns orgreater.

Although most of the discussion relates to oxidize ceramic compositions,the present invention encompasses both ceramic compositions also (theseinclude oxides, nitrides, borides, carbides, and any combination of theforegoing). The ceramic composition of the present invention has asubstantially thicker diffusion hardened zone than the Davidson-typeoxidized zirconium compositions. The diffusion zone of the compositionsof the present invention has a layered structure unlike the diffusionzone of the Davidson-type compositions of the prior art. The thicknessof the diffusion zone is at least equal to that of the ceramic layerformed on the surface of such an implant. This is accomplished byapplication of specific processes and the formation of a novelcomposition. FIG. 10 shows a comparison of oxygen concentration profileof the diffusion zone of Davidson-type oxidized zirconium compositionand that of a composition of the present invention. The oxygen richdiffusion zone in Davidson-type oxidized zirconium composition isbetween 1 to 2 microns. The oxygen concentration at the interface(between the oxide and diffusion hardened zone) is approximately equalto the solubility limit of oxygen in alpha zirconium which isapproximately 9% (w/w) or 30 atomic %. In the compositions shown in FIG.10, the oxygen rich diffusion zone is greater than 15 microns. FIG. 11shows a comparison of micro-hardness profiles of the Davidson-typeoxidized zirconium composition to one of the compositions of the presentinvention. The depth of hardening is significantly greater in thecomposition of the present invention compared to the Davidson-typecomposition. Two profiles (585° C.—10 hours and 685° C.—10 hours) appearto follow an exponential, error function type of profile. Samplesdiffusion hardened at 750° C. appear to follow a combination of uniformand error/exponential function. These combinations of differentfunctions appear to originate from the layered microstructure of thediffusion hardened zone and are related to the thickness of oxideretained on the surface. FIG. 12 shows anodized metallographiccross-sectional images of the Davidson-type oxidized zirconiumcompositions and new diffusion hardened compositions of the presentinvention. FIG. 12A shows the Davidson-type oxidized zirconiumcomposition. It is characterized by the oxide and a very smallunresolved diffusion hardened zone. The layered structure of thediffusion hardened zone of the present invention is absent in theDavidson-type composition. The total hardening depth of this compositionis approximately 7 microns. FIG. 12B illustrates the composition of thepresent invention. This particular composition has zirconium oxide andthe diffusion zone that is characterized by at least two layers. Thefirst layer is beneath the oxide and the second layer is beneath thefirst layer. Thickness of the second layer is less than the first layer.The total hardening depth is approximately 12 microns.

FIG. 12C shows another embodiment of the composition of the presentinvention. This particular composition has zirconium oxide on thesurface and the diffusion zone that is characterized by at least threelayers. The first layer is beneath the oxide, the second layer isbeneath the first layer and the third layer is beneath the second layer.The thickness of the first layer is greater than the second layer andthe thickness of the second layer is greater than that of the thirdlayer. The total hardening depth is approximately 30 microns. FIG. 12Dshows another embodiment of the composition of the present invention.This particular composition has zirconium oxide layer thickness which isless than 0.2 microns and difficult to resolve under an opticalmicroscope. The first layer is beneath the thin oxide. The second layeris beneath the first layer and the third layer is beneath the secondlayer. All the layers in this particular composition have similarthicknesses. In one aspect of this invention, the oxide ispreferentially retained on the surface (FIGS. 12B, 12C and 12D) duringthe vacuum treatment. This particular aspect leads to furtherdistinctions between the Davidson-type oxidized zirconium compositionand that of the present invention. The monoclinic content of thecomposition disclosed in this invention is typically greater than 96%(v/v). The typical monoclinic content of the Davidson-type oxidizedzirconium composition is less than 93% (v/v) (V. Benezra. S. Mangin, M.Treska, M. Spector, G. Hunter and L. Hobbs. Materials Research SocietySymposium Proceedings, Volume 550, Symposium held Nov. 30-Dec. 1, 1998,Boston, Mass., USA, L. Hobbs, V. Benezra Rosen, S. Mangin, M. Treska andG. Hunter, International Journal of Applied Ceramic Technology, 2(3),221-246, 2005 and Sprague, J., Aldinger, P., Tsai, S., Hunter, G.,Thomas, R., and Salehi, A., “Mechanical behavior of zirconia, alumina,and oxidized zirconium modular heads”, ISTA 2003, vol. 2. S. Brown, I.C. Clarke, and A. Gustafson (eds.), International Society for Technologyin Arthroplasty, Birmingham, Ala., 2003.). FIG. 13 shows the X-raydiffraction pattern of a Davidson-type oxidized zirconium and the X-raydiffraction pattern of the composition of the present invention. Thereflection of tetragonal phase is prominently present in Davidson-typecomposition whereas it is negligibly small in the composition disclosedin this invention. The typical monoclinic content of the composition ofthe present invention is equal to or greater than 96% (see Table 1). TheDavidson-type oxidized zirconium was produced by oxidizing at 635° C.for 75 minutes. One embodiment of the composition of the presentinvention was produced by oxidizing at 635° C. for 150 minutes andvacuum diffusion hardening at 685° C. for 10 hours at 10⁻⁴ torr. Theoxide was retained at the end of the process. The metallic hardenedlayer and part of the oxide were removed by mechanical polishing priorto x-ray diffraction analysis. The remaining phases are most likelycubic or tetragonal or amorphous or a combination thereof.

TABLE 1 Percent monoclinic content analysis of Davidson-type oxidizedzirconium and one of the compositions disclosed in this invention.Davidson-type Composition of oxidized the present Sample zirconiuminvention 1 84 ± 2 97 ± 1 2 82 ± 1 98 ± 2 3 82 ± 1 98 ± 1 Hobbs et. al.<93 — Sprague et. al. 88 ± 3 —

At room temperature, zirconium oxide is stable as a monoclinic phase. Itis believed that the prolonged treatment at elevated temperature led tothis distinction between the two compositions. Another distinction incomposition between the Davidson-type composition and that of thepresent invention is the structure of ceramic layer. In theDavidson-type oxidized composition a distinct secondary phase is seen inthe vicinity of the interface between the oxide and the substrate. Thissecondary phase extends from the substrate through the interface intothe oxide. This phase penetrates to an extent of approximately ¾ or lessof the oxide thickness. Only in rare occasions, this phase is seen atthe outer surface of the Davidson-type oxidized zirconium composition.In contrast to the Davidson-type oxidized composition, the compositionof present invention shows this distinct secondary phase through theentire thickness of the ceramic layer. In the Davidson-type oxidizedcomposition, this distinct secondary phase is visible only up to acertain depth in the oxide from the oxide-metal interface. FIG. 14 showsscanning electron microscope images of the cross-section showing oxideof Davidson-type oxidized zirconium composition and that of the presentinvention. In the Davidson-type composition of zirconium oxide, thesecondary phase is present from the oxide/metal interface to at most ¾of the oxide thickness (FIGS. 14A and 14B). Occasionally it is seen onthe surface of the oxide. This is consistent with that reported byBenezra et. al. and Hobbs et. al. Whereas, in the composition of thepresent invention, secondary phase is present through the entirethickness of the oxide (FIGS. 14C and 14D). Although not intending to bebound by theory, it is believed that this is due to the prolonged vacuumtreatment. FIG. 15 shows scanning electron microscope images of thesurface of the oxide. No secondary phase is seen on surface ofDavidson-type oxidized zirconium composition (FIG. 15A). The compositiondisclosed in this invention clearly shows presence of secondary phase onthe surface (FIG. 15B). It should be noted that this distinction isvisible when the ceramic layer is retained on the surface at the end ofthe vacuum treatment. If re-formation of the ceramic layer is carriedout after the diffusion treatment secondary phase may not be present upto the surface. As stated previously, underneath the ceramic layer is alayered structure of diffusion zone. The Zr-2.5Nb is comprised of twophases, alpha (hexagonal) and beta (cubic). The diffusion zone ispredominantly alpha phase (hexagonal). A minor amount of beta (cubic)phase (less than 7% (v/v)) can be present in the first layer ofdiffusion zone. The first layer is predominantly alpha phase and thevolume fraction of beta phase gradually increases in the diffusion layertowards the substrate. If the zirconium alloy is predominantly singlephase (alpha) then the beta phase in the diffusion zone will besignificantly less than it is in the substrate.

In one embodiment of the composition of the present invention, when theceramic layer is retained on the surface during the vacuum treatment,based on the pressure and temperature used, metallic hardened surface isformed on the ceramic layer along with the diffusion zone formedunderneath the ceramic layer. This metallic hardened zone is the resultof the reaction at the ceramic layer/vacuum interface. FIG. 16 showsratio of atomic concentration of oxygen to atomic concentration ofzirconium (O/Zr) of Davidson-type oxidized zirconium composition and oneof the compositions disclosed in this invention. If the organiccontamination on the surface is ignored, the O/Zr ratio of Davidson-typecomposition starts at 1.4 and seems to be constant through the thicknessevaluated in this analysis. For the new composition disclosed in thisinvention, O/Zr ratio starts at 0.3 and gradually increases to 1.2 inthe oxide. The top 0.2 micron layer shown in the image is the metallichardened layer described in this invention. This layer may or may not beretained on the final medical implant. Below this metallic hardenedlayer is the ceramic layer (in this case, an oxide) and below the oxideis the layered structure of diffusion zone. The composition of the oxidedisclosed in this invention appears to be slightly more oxygen deficientcompared to the Davidson-type composition. It should be noted that thisanalysis was carried out using x-ray photoelectron spectroscope (XPS).The surface was analyzed while being removed (sputtered) using an iongun. The depths are approximate and are based on the sputtering rate ofsilicon dioxide. XPS is sensitive to surface organic contamination(carbon-oxygen) and hence shows higher O/Zr ratio on the surface. It isreasonable to surmise that the top few layers (0.03 micron) are thesurface contaminants.

The diffusion-hardened ceramic layers of this invention are produced byemploying three processes. All processes can be performed in a single ormultiple steps. The processes are (1) ceramic layer formation (i.e.,oxidation, nitridation, boridation, carburization, or any combinationthereof), (2) diffusion hardening, and optionally, (3) ceramic layerformation. If the ceramic layer is retained on the surface duringdiffusion hardening, process 1 and 2 may be sufficient. If the finalapplication is such that a ceramic layer is not required on the surface,a temperature and time can be chosen in such a way that process 2 willdissolve the ceramic layer completely. Alternatively, the surfaceceramic layer may be removed by mechanical, chemical or electrochemicalmeans. When the ceramic layer is retained on the surface it may form ametallic hardened layer on the oxide. This film may or may not beremoved for the final product. If the ceramic layer is completelydissolved into the substrate and re-formation of the ceramic layer isdesired then a diffusion profile is obtained which will produce a highintegrity and defect-free ceramic layer during the ceramic layerformation process. This diffusion profile can be an exponentialfunction, an error function, a uniform, or any sequential combinationthereof (FIG. 6, Profiles 1 to 3). It should be noted that some of thesefunctions may also be attributed to be linear or higher orderpolynomials. It should be noted that the combination of diffusionprofile and retained oxide is obtained through careful control of time,temperature and pressure during the ceramic layer formation process andthe diffusion hardening process.

For Zr-Nb-based alloys, the damage-resistant implant is such that it hasceramic layer thicknesses ranging from 0.1 to 25 microns and a diffusionhardened zone (DHZ) significantly greater than 2 micron. The DHZ can be70 micron or greater. The DHZ is defined as the region which hashardness at least 1.1 times of the substrate hardness.

There are three general methods to produce the composition of thepresent invention. It should be understood that variations by way ofsubstitutions and alterations from these general methods which do notdepart from the spirit and scope of the invention are understood to bewithin the scope of the invention. In this way, the general methodsdescribed below are merely illustrative and not exhaustive. In each ofthe examples provided, the ceramic layer formation steps are oxidationsteps (thereby producing ceramic oxides). It should be understood thatthese steps are not limited to oxidation and the formation of ceramicoxides; in addition to or in the alternative of, an oxidation step, onemay use a carburization step, a boridation step, a nitridation step, orany combination thereof (including a combination of oxidation and one ormore other steps). In this way, the ceramic so produced can be any oneor, or a combination of an oxide, nitride, boride, and carbide.

In Method A, the ceramic oxide and a thick diffusion hardened zone onthe damage-resistant surface is formed by carrying the following processsteps:

1. Ceramic Layer Formation. Oxidation by diffusion of oxygen in air attemperature less than 700° C. for times greater than 5 minutes. Theoxidation time can be approximated by parabolic relationship of time andoxide thickness (x²=kt, where k is a constant, t is time and x isthickness of the oxide, k is function of temperature). In certain casesa cubic or higher order polynomial relationship may also be employed.

2. Diffusion Hardening. Treating under vacuum or under inert gas theabove said implant at a temperature range from 500° C. to 1000° C. for aperiod of greater than 1 hour in vacuum at a pressure less thanatmospheric (typically less than 10⁻² torr). This step either partiallyor completely dissolves the oxide layer formed in step 1. The oxygenatoms thus released are driven deeper into the alloy substrate,hardening the material. The time and temperature required to obtain acertain diffusion hardening depth can be estimated from anerror-function relationship. Hardness at depth d (H_(d)) is given by:

$H_{d} = {H_{i} + {\left( {H_{i} - H_{o}} \right){{erf}\left\lbrack \frac{- d}{2\sqrt{Dt}} \right\rbrack}}}$

where, H_(i) is the hardness at the interface, H_(o) is the hardness ofthe bulk substrate significantly away from the diffusion zone. D isdiffusivity of diffusing species at the vacuum treatment temperature andt is time of treatment. “erf” is the error function. All the parametersshould be used in consistent units. The diffusivity of oxygen can beobtained from the published literature. In this relationship, it isassumed that the hardness is directly proportional to oxygen at allconcentration levels, and diffusivity of diffusing specie is independentof concentration. This is a simplistic view to approximately estimatethe depth of hardening. Those skilled in the art can hypothesizedifferent relationships of diffusing specie and the hardness and mayobtain a different relationship but the overall shape and profile willfollow that described in this invention. As an example, if therelationship is exponential or combination of uniform and exponential orerror function, then the depth estimation will be inaccurate using theabove said equation. An example of same is shown in FIG. 17. Sample inFIG. 17A was oxidized at 635° C. for 75 minutes and then subsequentlydiffusion hardened at 685° C. for 10 hrs. The oxide was retained on thissample after the vacuum diffusion treatment. An error function fit seemsto be adequate. Sample in FIG. 17B was oxidized at 635° C. for 75minutes and then was diffusion hardened at 750° C. for 20 hrs. A verysmall fraction of oxide was retained on the surface. An error functionfit is not adequate for this particular sample. It seems that sequentialcombination of error function and uniform fit may model the hardeningbehavior.

3. Optional Ceramic Layer Formation. Optionally, the implant issubsequently oxidized again at temperature less than 700° C. in air fortimes greater than 5 minutes. As shown in FIGS. 6 and 7, a suitablehardness profile prior to oxidation is essential to produce highintegrity substantially defect-free oxide.

Ceramic layer formation and diffusion hardening at temperatures lessthan 700° C. helps to preserve the microstructure of the substrate. FIG.18 shows the substrate microstructure of samples diffusion hardened atdifferent temperatures. The grain size of the as-received bar stock isless than 1 micron (FIG. 18A). The microstructure shows orientation ofthe grains along the rolling direction. The grain size of the samplesdiffusion hardened at 585° C. show slight coarsening (FIG. 18B). Theorientation of the microstructure is still preserved. FIG. 18C showsgrain size of the sample diffusion hardened at 685° C. for 20 hours. Thegrain size shows noticeable coarsening compared to as-received barstock. The orientation of the grains is still present. FIG. 18D showsmicrostructure of samples diffusion hardened at 750° C. for 20 hours.There is significant coarsening of the grains. The orientation of thegrains has disappeared and the grains have become equiaxed. The size ofthe grains is greater than 1 micron. FIG. 18E shows microstructure ofsamples diffusion hardened at 850° C. for 22 hours. Significantcoarsening of the grains can be seen. The size of the grains is above 10microns. Alternatively, the second step may be carried out at atemperature and time such that part of the oxide formed in step 1 isretained on the surface. The third step of ceramic layer formation maybe altogether eliminated if any remaining ceramic layer is sufficient.It should be noted that when the ceramic layer is retained on thesurface, a thin metallic hardened film forms on the surface. Thecomposition of the film is shown FIG. 16. This film may be retained onthe surface or can be polished by mechanical, chemical orelectrochemical means if desired. Alternatively, the second step ofdiffusion hardening is carried out in an inert atmosphere such ascomposed of argon (or other inert gas) with partial pressure of oxygen(or other diffusion hardening species) in the system typically less than0.2×10⁻² torr and temperature range from 500° C. to greater than 800° C.Alternatively, if re-formation of ceramic layer is desired as a thirdstep, an adequate diffusion profile is obtained to produce a highintegrity, predominantly defect-free ceramic layer (FIGS. 6 and 7).

In Method B, the ceramic oxide and a thick diffusion hardened zone onthe damage-resistant surface is formed by carrying the following processsteps:

1. Ceramic Layer Formation. Oxidation by diffusion of oxygen in air attemperature range of 500° C. to 1000° C. (preferably less than 700° C.)for times greater than 5 minutes. The oxidation time can be approximatedby parabolic relationship of time and oxide thickness (x²=kt, where k isa constant, t is time and x is thickness of the oxide, k is function oftemperature). In certain cases a cubic or higher order polynomialrelationship may also be employed.

2. Diffusion Hardening. Treating under vacuum (i.e., pressure less thanabout 10⁻² torr) or under inert gas the above said implant at atemperature of less than 700° C. The exact temperature and time arechosen such that a desired oxide thickness remains on the surface afterthe vacuum treatment step is completed. This step likely partiallyconsumes the oxide layer formed in step 1. The oxygen atoms thusreleased are driven deeper into the alloy substrate, hardening thematerial. The diffusion hardening depth can be estimated from anerror-function relationship. Hardness at depth d (H_(d)) is given by:

$H_{d} = {H_{i} + {\left( {H_{i} - H_{o}} \right){{erf}\left\lbrack \frac{- d}{2\sqrt{Dt}} \right\rbrack}}}$

where, H_(i) is the hardness at the interface, H_(o) is the hardness ofthe bulk substrate significantly away from the diffusion zone. D isdiffusivity of diffusing species and t is time of treatment. “erf” isthe error function. All the parameters should be used in consistentunits. The diffusivity of oxygen can be obtained from the publishedliterature. In this relationship, it is assumed that the hardness isdirectly proportional to oxygen at all concentration levels, anddiffusivity of diffusing specie is independent of concentration. This isa simplistic view to approximately estimate the depth of the hardening.Those skilled in the art can hypothesize different relationships ofdiffusing specie and the hardness and may obtain a differentrelationship but the overall shape and profile will follow thatdescribed in this invention. It should be noted that this relationshipis an approximate way to estimate the depth of hardening. If the profileis exponential or combination of uniform and exponential or errorfunction, then the depth estimation using the equation above will beinaccurate. An example of same is shown in FIG. 17. Sample shown in FIG.17A was oxidized at 635° C. for 75 minutes and then subsequentlydiffusion hardened at 685° C. for 10 hours. Oxide was retained on thesurface. An error function fit seems to be adequate. Sample in FIG. 17Bwas oxidized at 635° C. for 75 minutes and then was diffusion hardenedat 750° C. for 20 hours. A small fraction of oxide was retained on thesurface. An error function fit is not adequate for this particularsample. It seems that sequential combination of error function anduniform fit may model the hardening behavior.

3. Optional Ceramic Layer Formation. Optionally, the implant issubsequently oxidized again at temperature less than 700° C. in air fortimes greater than 5 minutes. As shown in FIGS. 6 and 7, a suitablehardness profile prior to oxidation is essential to produce highintegrity substantially defect-free oxide.

By vacuum (or inert gas) treating at lower temperatures a desired oxidethickness remains on the surface and promotes the preservation of themicrostructure of the substrate. FIG. 18 shows the substratemicrostructure of samples diffusion hardened at different temperatures.The grain size of the as-received bar stock is less than 1 micron (FIG.18A). The microstructure shows orientation of the grains along therolling direction. The grain size of the samples diffusion hardened at585° C. show slight coarsening (FIG. 18B). The orientation of themicrostructure is still preserved. FIG. 18C shows grain size of thesample diffusion hardened at 685° C. for 20 hours. The grain size showsnoticeable coarsening compared to as-received bar stock. The orientationof the grains is still present. FIG. 18D shows microstructure of samplesdiffusion hardened at 750° C. for 20 hours. There is significantcoarsening of the grains. The orientation of the grains has disappearedand the grains have become equiaxed. The size of the grains is greaterthan 1 micron. FIG. 18E shows microstructure of samples diffusionhardened at 850° C. for 22 hours. Significant coarsening of the grainscan be seen. The size of the grains is above 10 microns. Alternatively,if re-formation of ceramic layer is desired as a third step, an adequatediffusion profile is obtained to produce a high integrity, predominantlydefect-free ceramic layer (FIGS. 6 and 7).

In Method C, the ceramic oxide and a thick diffusion hardened zone onthe damage-resistant surface is formed by carrying the following processsteps:

1. Ceramic Layer Formation. Oxidation by diffusion of oxygen in air attemperature less than 700° C. for times greater than 5 minutes. Theoxidation time can be decided based on the parabolic relationship oftime and oxide thickness (x²=kt, where k is a constant, t is time and xis thickness of the oxide, k is function of temperature). In certaincases a cubic relationship may also be employed.

2. Diffusion Hardening. Treating under vacuum (i.e., pressure less thanabout 10⁻² torr) or under inert gas the above said implant at atemperature of less than 700° C. The exact temperature and time ischosen such that a desired oxide thickness remains on the surface afterthe vacuum treatment step is completed. This step likely partiallyconsumes the oxide layer formed in step 1. The oxygen atoms thusreleased are driven deeper into the alloy substrate, hardening thematerial. The time and temperature required to obtain a certaindiffusion hardening depth can be estimated from an error-functionrelationship. Hardness at depth d (H_(d)) is given by:

$H_{d} = {H_{i} + {\left( {H_{i} - H_{o}} \right){{erf}\left\lbrack \frac{- d}{2\sqrt{Dt}} \right\rbrack}}}$

where, H_(i) is the hardness at the interface, H_(o) is the hardness ofthe bulk substrate significantly away from the diffusion zone, D isdiffusivity of diffusing species and t is time of treatment. “erf” isthe error function. All parameters should be used in consistent units.The diffusivity of oxygen can be obtained from the published literature.In this relationship, it is assumed that the hardness is directlyproportional to oxygen at all concentration levels, and diffusivity ofdiffusing specie is independent of concentration. This is a simplisticview to approximately estimate the depth of hardening. As an example, ifthe relationship is exponential or combination of uniform andexponential or error function, then the depth estimation will beinaccurate. An example of same is shown in FIG. 17. Sample in FIG. 17Awas oxidized at 635° C. for 75 minutes and then subsequently diffusionhardened at 685° C. for 10 hours. Oxide was retained on the surface. Anerror function fit seems to be adequate. Sample in FIG. 17B was oxidizedat 635° C. for 75 minutes and then was diffusion hardened at 750° C. for20 hours. A small fraction of oxide was retained on the surface. Anerror function fit is not adequate for this particular sample. It seemsthat a sequential combination of error function and uniform fit maymodel the hardening behavior.

3. Optional Ceramic Layer Formation. Optionally, the implant issubsequently oxidized again at temperature less than 700° C. in air fortimes greater than 5 minutes. As shown in FIGS. 6 and 7, a suitablehardness profile prior to oxidation is essential to produce highintegrity, substantially defect-free oxide.

By performing the ceramic layer formation and diffusion hardening(vacuum or inert gas treatment) steps at lower temperatures preservationof the microstructure of the substrate is achieved and a desired ceramiclayer thickness remains on the surface as shown in FIGS. 12 and 18. Thecombination of the ceramic layer formation and diffusion hardening stepsdescribed results in a significantly thicker diffusion hardened zone(greater than 2 microns and preferably greater than 5 microns) incomparison to Davidson-type diffusion hardened oxide and/or nitridecompositions. Additionally, the totality of the ceramic layer and thediffusion hardened zone is 5 microns or greater. These properties resultin a more damage resistant and wear resistant surface, among otheradvantages. The properties of the new composition makes it applicable tohard-on-hard medical implant applications. Non-limiting examples of suchinclude knee and hip prostheses having one surface of the newcomposition articulating against another surface of the new composition.

It should be understood that the temperature and time parameters can bevaried from those provided above, particularly in the case of differentsubstrate compositions. Additionally, the processes may be carried outin a controlled atmosphere. Illustrative but non-limiting examples of acontrolled atmosphere include, controlled oxygen and nitrogen partialpressure, oxygen plasma, in the presence of water gas reactions, in thepresence of reactive gases such as oxygen and ozone in the presence ofinert gases such as argon and nitrogen, in the presence of oxidizing orreducing salts, in the presence of glasses etc. Examples of inert gasesinclude nitrogen, argon, etc. Examples of reactive gases includehydrogen, methane, other hydrocarbons, etc. Other controlled atmosphereconditions, known to those of skill in the art are also included. Thegoal is to form the composition under conditions that do notsignificantly change the microstructure of the substrate alloy.

Alternatively, the process of ceramic layer formation and diffusionhardening can be carried out in an atmosphere that is lean in oxygen (orother ceramic forming species) content (e.g., partial pressure of oxygenless than 0.05 bar). Alternatively, the process can be carried out in asingle step comprising of all the above steps in one process.Alternatively, the process can be carried out in ozone atmosphere or anatmosphere whose oxidation potential is controlled by water-gas reactionsuch as CO₂+H₂═H₂O+CO or using controlled moisture in an inert gasessuch as but not limited to helium, nitrogen, argon and krypton.

Alternatively, the ceramic layer formation and diffusion hardening canbe carried out in two steps that do not change microstructure of thesubstrate alloy significantly. The process of ceramic layer formationand diffusion hardening can be carried out in a two step process. In thefirst step, the alloy is treated with ceramic forming species at atemperature above 700° C. for a period of greater than 12 hours thatforms a thicker diffusion zone along with a cracked ceramic layer or thealloy is diffusion hardened as described in methods A. B and C. In asecond step, the ceramic layer or part of the diffusion zone is removedby mechanical, chemical or electrochemical means and the alloy issubsequently treated to form a ceramic layer at a lower temperature andtime to form an adherent ceramic layer with an already formed diffusionzone and thus producing the damaged resistant implant.

Alternatively, the substrate material is first diffusion hardened usinga lean concentration of diffusion hardening species and then a ceramiclayer is formed (using a more concentrated dose of ceramic-formingspecies to form the ceramic layer).

A two step process can be used. In the first step the material isdiffusion-hardened (oxygen, carbon, boron, or nitrogen) in controlledconditions in which the partial pressure of the hardening species arelean enough not to form stable ceramic compounds with the alloy. Thediffusion zones can be controlled as described above. This is followedby oxidation, carburization, nitridation, borization or any combinationthereof as described above.

The damage-resistant implant is produced by forming the ceramic layer ata temperature preferably ranging from 500° C. to greater than 1000° C.for a time preferably ranging from 5 minutes to greater than 6 hours. Itis preferred that the ceramic formation temperature be under 700° C. topromote preservation of the substrate microstructure. The time andtemperature may be determined from the Arrhenius and parabolicrelationship amongst the ceramic layer thickness, diffusion-hardenedzone thickness, and temperature. Vacuum or inert gas treatment(diffusion hardening) is preferably performed at a temperaturepreferably ranging from 500° C. to greater than 1000° C. for a timepreferably ranging from 15 minutes to greater than 30 hours. It ispreferred that the diffusion hardening treatment temperature be under700° C. to preferentially preserve any of the ceramic oxide formed instep 1 and also to promote preservation of the substrate microstructure.An optional step of re-formation of ceramic layer may be performed afterthe initial ceramic layer formation step if additional ceramic layergrowth is desired.

The resulting surface composition can be subject to a variety of surfacepreparation techniques after the step of diffusion-hardening to form theadherent oxide. Such techniques include, but are not limited to, thosetechniques known in the art to be applicable to diffusion-hardenedsurfaces. It is expected that other, more rigorous techniques areapplicable to the composition of the present invention due to itsgreater degree of damage resistance.

In the composition used in the medical implant of the present invention,the totality of the thickness of the ceramic layer and the diffusionhardened zone is greater than 5 microns, and preferably greater than 10microns. Because the ceramic layer may or may not be present (it canrange in thickness from 0 to 25 microns), this requirement may be met bya diffusion hardened zone of a thickness of greater than 5 microns (andpreferably greater than 10 microns) with no ceramic layer above it or aninfinitesimally small ceramic layer above it. Where both layers arepresent, the ceramic layer is on the surface and is above the diffusionhardened zone. While the diffusion hardened zone is one of the twoaforementioned layers, the diffusion hardened zone itself consists of atleast two distinct layers layer (visible by metallographic analysis).The first layer of the diffusion hardened zone has a relatively highconcentration of diffusion hardening species (higher than that of thebulk substrate zirconium or zirconium alloy) and may be saturated withthe diffusion hardening species. The zirconium in the first layer ispredominantly alpha phase zirconium (the first layer of the diffusionhardened zone is that layer which is closest to the ceramic layer, or,where the ceramic layer is absent, the first layer is that layer whichis nearest to the surface of the composition). The second layer is belowthe first layer and has a lower content of diffusion hardening speciesthan the first layer. The diffusion hardened zone has a diffusionhardening species concentration profile such that, in one or morecross-sections of the diffusion hardened zone, the concentration ofdiffusion hardening species decreases as either an error function, anexponential function, a near uniform distribution, or sequentialcombinations thereof. Where combinations of functional profiles arereferred to, it should be understood that such combinations aresequential combinations and do not refer to the superposition of thevarious functional profiles. Where the diffusion hardened layer is verythick due to the use of long formation times, the distribution mayapproach a uniform distribution in at least some sections of thediffusion hardened zone.

The layered structure of the diffusion hardened zone can be detected bymetallographic analytical techniques known to those of ordinary skill inthe art. These include, but are not limited to, anodization, heattinting, x-ray diffraction, Auger spectroscopy, depth profiling, etc.

As described above, the process can be used for an extended period toform a thick cracked ceramic layer and a thick diffusion hardened layer.The cracked ceramic layer can then be removed to retain the diffusionhardened layer for subsequent re-formation of another ceramic layer.

The new composition has application in medical implants of allvarieties. It is expected to be particularly beneficial for use inarticulating implants, such as, but not limited to hip and kneeimplants. Use of such product in other biomedical applications suchspinal devices, small joints, shoulder joints, etc.

Resulting medical implants comprising diffusion-hardened ceramic layersof the variety described herein are heated to desired temperatures usingelectric heating, radiative heating, induction heating or usingtechniques such as spark plasma sintering or field assisted sintering.This is accomplished by use of an alloy of Ti, Zr and Nb that is capableof producing thicker totality of hardened zones (ceramic layer and thickdiffusion hardened zone) that is produced by specific processes.

The present composition will be applicable for any and all medicalimplants, but in particular for articulating medical implants such as,but not limited to, hip, knee, shoulder, elbow orthopedic implants, etc.Vertebral implants are also amenable to the present invention. Thepresent invention also finds applicability to any and allnon-articulating medical implants. The improved damage resistance isseen in comparison to the diffusion hardened oxides of theDavidson-type, such as those described in U.S. Pat. No. 5,037,438 toDavidson and U.S. Pat. Nos. 6,447,550; 6,585,772 and pending U.S.application Ser. No. 10/942,464 to Hunter.

Although the present invention and its advantages have been described indetail, it should be understood that various changes, substitutions andalterations can be made herein without departing from the spirit andscope of the invention as defined by the appended claims. Moreover, thescope of the present application is not intended to be limited to theparticular embodiments of the process, machine, manufacture, compositionof matter, means, methods and steps described in the specification. Asone of ordinary skill in the art will readily appreciate from thedisclosure of the present invention, processes, machines, manufacture,compositions of matter, means, methods, or steps, presently existing orlater to be developed that perform substantially the same function orachieve substantially the same result as the corresponding embodimentsdescribed herein may be utilized according to the present invention.Accordingly, the appended claims are intended to include within theirscope such processes, machines, manufacture, compositions of matter,means, methods, or steps.

The invention claimed is:
 1. A medical implant comprising: a firstimplant portion having a first articulating surface, the first implantportion comprising: a substrate comprising a biocompatible metal ormetal alloy; a diffusion hardened zone in contact with said substrate,said diffusion hardened zone comprising zirconium or zirconium alloy anda diffusion hardening species, said diffusion hardened zone having athickness of greater than 5 microns; and, a substantially defect-freeceramic layer in contact with and overlaying said diffusion hardenedzone and comprising a surface of said medical implant, said ceramiclayer ranging in thickness from 0.1 to 25 microns; and, wherein thetotal thickness of the ceramic layer and the diffusion hardened zone isgreater than 5 microns; and a second implant portion having a secondarticulating surface arranged and configured to contact the firstarticulating surface so that the second implant portion can articulaterelative to the first implant portion, wherein the second articulatingsurface comprises a soft polymer; wherein said biocompatible metal ormetal alloy is zirconium or zirconium alloy; and wherein the diffusionhardened zone comprises a layered structure comprising at least twodistinct layers under metallographic analysis, the layered structurecharacterized by: a first layer directly below the ceramic layer,wherein the first layer is predominantly alpha phase zirconium; aninterface between the first layer and the ceramic layer; and a secondlayer directly below the first layer.
 2. The medical implant of claim 1,wherein said soft polymer is selected from one of a polyethylene, anultra high molecular weight polyethylene (UHMWPE), a cross-linkedpolyethylene (XLPE), and polyurethane.
 3. The medical implant of claim1, wherein the medical implant is selected from one of a hip implant, aknee implant, a shoulder implant, an elbow implant, and a spinalimplant.
 4. The medical implant of claim 1, wherein said diffusionhardened zone has a thickness of greater than 10 microns.
 5. The medicalimplant of claim 1, wherein said diffusion hardening species is oxygenand the diffusion hardened zone has a concentration of oxygen whichdecreases in the direction of the substrate, said decrease of oxygenconcentration being defined by a function selected from the groupconsisting of an error function, an exponential function, a near uniformdistribution function, and any sequential combination thereof.
 6. Themedical implant of claim 1, wherein said diffusion hardened zone has ahardness profile which is defined by a function selected from the groupconsisting of an error function, an exponential function, a near uniformdistribution function, and any sequential combination thereof.
 7. Themedical implant of claim 1, wherein said substrate comprises an alloy ofzirconium and niobium and has a niobium content of at least 1% (w/w). 8.The medical implant of claim 1, further comprising an oxygen-containingzirconium alloy overlaying said ceramic layer on the surface of saidimplant, said alloy being in the metallic state.
 9. The medical implantof claim 1, wherein said substrate further comprises titanium, tantalum,hafnium, niobium, and any combination thereof; and, said diffusionhardening species is selected from the group consisting of oxygen,nitrogen, boron, carbon, and any combination thereof.
 10. A medicalimplant comprising: a first implant portion, said first implant portionhaving a first articulating bearing surface; and a second implantportion, said second implant portion having a second articulatingbearing surface; wherein said first articulating bearing surface andsaid second articulating bearing surface are capable of contacting oneanother; wherein said first articulating bearing surface comprises: asubstantially defect-free ceramic layer overlaying and in contact with adiffusion hardened zone, wherein said diffusion hardened zone has athickness of at least 2 microns; wherein the thickness of the diffusionhardened zone is at least equal to or greater than the thickness of thesubstantially defect free ceramic layer; and wherein said secondarticulating bearing surface comprises a soft polymer; wherein thediffusion hardened zone comprises: a first layer above a second layer,the thickness of said first layer is greater than the thickness of saidsecond layer.
 11. The medical implant of claim 10, wherein said softpolymer is selected from one of a polyethylene, an ultra high molecularweight polyethylene (UHMWPE), a cross-linked polyethylene (XLPE), andpolyurethane.
 12. The medical implant of claim 10, wherein the medicalimplant is selected from one of a hip implant, a knee implant, ashoulder implant, an elbow implant, and a spinal implant.
 13. Themedical implant of claim 10, wherein the first implant portion comprisesa material selected from the group consisting of zirconium, zirconiumalloy, titanium, tantalum, hafnium, niobium, and any combinationthereof.
 14. The medical implant of claim 10, wherein said diffusionhardened zone has a hardness profile defined by a function selected fromthe group consisting of an error function, an exponential function, anear uniform distribution function, and any sequential combinationthereof.
 15. The medical implant of claim 10, wherein said diffusionhardened zone comprises a diffusion hardening species selected from thegroup consisting of oxygen, nitrogen, boron, carbon, and any combinationthereof.
 16. The medical implant of claim 10, wherein said diffusionhardened zone further comprises a third layer below said second layer,the thickness of said first layer is greater than the thickness of saidthird layer.
 17. The medical implant of claim 10, further comprising asubstrate below said diffusion hardened zone having a grain size betweenabout 1 micron and about 10 microns.